Not Applicable.
Not Applicable.
The present embodiments relate to spread spectrum communications systems and, more particularly, to the assignment of spread spectrum signals to demodulation elements in a communications receiver.
Wireless communications have become very prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (xe2x80x9cCDMAxe2x80x9d) cellular communications. In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a xe2x80x9ccellxe2x80x9d, or a sector of a cell, as that term is used in the art. Due to movement of the user station and the fixed location of other cells with corresponding base stations, the user station may communicate with one or more than one base station at a time in the same frequency band, and this operation is typically referred to as soft handoff. As a result and in a simple example, this dual base station, user station communication is achieved by two different xe2x80x9cpathsxe2x80x9d of forward/reverse communication, that is, a first path from the user station to the first base station and a second path from the user station to the second base station and a first path from the first base station to the user station and a second path from the second base station to the user station.
In the actual reality of CDMA communications, and due to the wireless medium, a same transmitted communication from a base station to a user station may arrive at the user station at multiple and different times, where each different arriving signal is said to travel along a channel and arrive as a different xe2x80x9cpath.xe2x80x9d This is because the transmitted signal from the base station is reflected by objects such as the ground, mountains, buildings, and other things which it contacts. These multiple signals are referred to in the art as multiple paths or multipaths. Thus, several multipaths may eventually arrive at the user station but the channel traveled by each may cause each path to have a different phase, amplitude, and signal-to-noise ratio (xe2x80x9cSNRxe2x80x9d). Accordingly, for one communication between one base station and one user station, each multipath is a replica of the same user information, and each path is said have time diversity relative to other mulitpath(s) due to the difference in arrival time which causes different (uncorrelated) fading/noise characteristics for each multipath. However, as detailed later, other types of signals with the same information also may arrive at the receiver at different times and, thus, to distinguish these other signals from multipath signals then for purposes of the remainder of this document the time diversity among different multipaths is referred to as mulitpath diversity.
Although multipaths carry the same user information to the receiver they may be separately recognized by the receiver based on the timing of arrival of each multipath. More particularly and as known in the art, CDMA communications are modulated using a spreading code which consists of a series of binary pulses, and this code runs at a higher rate than the symbol data rate and determines the actual transmission bandwidth. In the current industry, each piece of CDMA signal transmitted according to this code is said to be a xe2x80x9cchip,xe2x80x9d where each chip corresponds to an element in the CDMA code. Thus, the chip frequency defines the rate of the CDMA code. Given the use of transmission of the CDMA signal using chips, then multipaths separated in time by more than one of these chips are distinguishable at the receiver because of the low auto-correlations of CDMA codes as known in the art.
By way of further introduction, note that other types of signal diversities are known in the art, where one such diversity is referred to as base station diversity. More particularly, recall from above that a user station may over a common time period receive the same user information from two different base stations. Thus, the information received from one base station is said to have base station diversity with respect to the information received from another base station. Again, so long as these base station diverse signals are adequately separated in time or use different codes, then they are distinguishable from one another at the receiver.
In the prior art, after identifying received paths the receiving station sets forth to demodulate certain ones of the paths. More particularly, it is known in the art to include a demodulation circuit in a CDMA receiver, where such a circuit commonly includes more than one demodulation element. For example, one such type of demodulation circuit is a Rake combiner, which is given its name in the art to suggest the notion of a yard rake having xe2x80x9cfingersxe2x80x9d and where each finger corresponds to a different demodulation element. In contemporary systems, such a demodulation circuit may include on the order of four or six different demodulation elements, where each element is capable of concurrently demodulating a received path that is assigned to the element. Accordingly, in the prior art as different multipath or base station diversity signals are received, the receiver selects which ones to assign to its available demodulation elements. Thus, this requires a determination if any such demodulation elements are not currently in use, of if they are in use, whether there should be a reassignment whereby a newly-received path pre-empts that use to take the place of a path that was earlier assigned to a demodulation element and is currently being demodulated by that element.
The objective of the process of selecting certain paths for demodulation is typically to optimize a performance parameter such as the frame error rate (FER) or the symbol error rate (SER), or to enhance the reliability of the communication link by reducing the probability of an outage. As an example, a method for demodulation element assignment in a CDMA communications system is disclosed in U.S. Pat. No. 5,490,165 entitled xe2x80x9cDEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALSxe2x80x9d, issued Feb. 6, 1996 (the ""165 patent). The ""165 patent provides different demodulation assignment methods based on whether the receiver is the user station or the base station, each of which is separately discussed below.
In the ""165 patent, the demodulation element assignment method for the user station emphasizes base station diversity. More particularly, the ""165 patent approach requires that the user station always has a demodulation element assigned to at least one path originating from the signal transmitted by each different base station. Thus, if the user station receives a path from a new base station (i.e., one which is not currently being demodulated by the user station), then the user station unconditionally assigns a demodulation element to the received path. This assumes that the user station communicates with more than one base station at that particular time instant, that is, that the user station is in soft handoff. If the user station is not in soft handoff or if it has a demodulation element assigned to each base station participating in soft handoff, the remaining unassigned demodulation elements are assigned to paths having the largest SNR (or signal-to-interference ratio (SIR)). Also, as long as every different base station communicating with the user station has at least one demodulation element assigned, all but the strongest path (i.e., largest SNR or SIR) from each base station may be dismissed and the demodulation elements be reassigned to paths with the largest SNRs.
Having introduced soft handoff above, a few other prior art observations relating to soft handoff are now noted. The assignment of a first path from a new base station to a demodulation element of a user station initiates the soft handoff process. This is typically a decision made by the system controller. The user station is instructed through the base station currently supporting the communication that a new base station begins transmission to the user station. The user station may then assign one or more paths from the new base station to demodulation elements.
Returning to the ""165 patent, the demodulation element assignment method for the base station assumes that the only type of diversity for incoming paths is multipath diversity. Given this assumption, for each signal received by the base station from a user station the demodulation elements assignments are entirely based on the received path SNR. Thus, the paths with the larger SNRs are assigned to the demodulation elements. Also, at both the user station and the base station a typical hysteresis method as commonly known in the literature was assumed.
While the ""165 patent provides methods of demodulation element assignment for either a base station or a user station, the present inventor has recognized that it has various drawbacks. As one such drawback, for the user station the unconditional assignment of the ""165 patent based on base station diversity (i.e., to an arriving signal from a new base station) may in some instances cause a less than optimal use of the demodulator. For example, the demodulation element assigned to the second base station path is occupied and not available for demodulating paths from the first base station, even though those paths may have a substantially greater SNR or other reason rendering them more beneficial to the receiver as opposed to a path from the second base station.
As another drawback, the assignment method of the ""165 patent provides for at most only one type of diversity for the user station (i.e., base station diversity), and does not at all consider diversity for signals received by the base station (i.e., it only evaluates SNR). However, as has been recognized in the art in areas other than demodulation, numerous other types of diversity also exist in CDMA signals. Thus, due to these other diversity types, again there is a chance for different paths to be received by a same station during a same time period where the signals include replica user information but where certain characteristics of the path carrying the information differ in one or more respects from other paths received during the same time period. For example, often a base station uses multiple antennas where each antenna transmits the same signal to the user station, thereby causing multiple paths to arrive at the user station. Another source of multiple signals may occur when the user station discriminates arriving signal paths using more than one antenna (i.e., at least one path per receiving antenna). Still other types of diversity are known in the art, such as angle diversity (e.g., paths arriving along different electromagnetic planes) and code diversity (signals transmitted with different spreading codes), and still others may be ascertained by one skilled in the art. Lastly, note also that some or all of the above sources of multiple paths also may appear in the reverse communication link, that is, for the signal transmitted by a user station and received by a base station. Each of these additional types of diversity is not expressly considered in the method of the ""165 patent; moreover, these different diversities may or may not predictably affect SNR and, thus, they are not necessarily implicitly considered in the method of the ""165 patent as well. In the present inventive embodiments, however, and as explored below, the present inventor has recognized that some or all of these diversity types may prove useful in the process of selecting certain paths for demodulation, as is achieved by the preferred embodiments discussed below.
In the preferred embodiment, there is a method of operating a spread spectrum communications receiver. The method demodulates a current path group with a demodulator, wherein the current path group comprises one or more current paths and wherein the demodulating step comprises demodulating the one or more current paths with a respective one or more demodulation elements. The method also determines one or more new path groups (survey groups) at the receiver, wherein each survey group comprises one or more new paths and zero or more current paths. Still further, the method determines a quality measure of the current path group and of a plurality of survey groups. Each of the survey groups comprises a different combination of one or more new paths and zero or more current paths. Further, the method selectively assigns a selected one of the survey groups to respective elements of the demodulator in place of the current path group in response to a comparison of the quality measure of the selected one of the survey groups with the quality measure of the current path group when the survey group has a larger quality measure than the current group. The selected survey group also has the largest quality measure among all survey groups. Finally, the quality measure is responsive to the power or signal-to-noise ratio of each path in a group and multiple forms of diversity within a group. Other circuits, systems, and methods are also disclosed and claimed.